After years of research and treatment, apple scab, caused by Venturia inaequalis (CKE.) Wint., is economically the worst disease of apple trees (Malus domesticus L.) worldwide (Agrios, 1988). Much research has been devoted to the control of primary infections, and it has yielded very costly and ecologically questionable spraying schedules (Funt, 1990). These sprays represent an appreciable cost to growers and can have a substantial indirect impact on the environment. Development of fungicide resistance in the pathogen population is also threatening apple production. Thus it is essential to develop an ecologically and environmentally friendly alternative control strategy for apple scab.
In cold temperature regions, the fungal pathogen V. inaequalis overwinters as a saprophyte and to a significant extent only as incipient pseudothecia (sexual structures) in fallen apple leaves on the orchard floor. Pseudothecia, initiated during fall or winter, mature in the spring to produce ascospores which serve as primary inoculum for the initial infections (Ellis 1990). Thus, the overwintering stage is one weak link in the life cycle of the fungus. If the pathogen could be killed or seriously weakened in the leaf litter, the primary inoculum available in the spring would be substantially reduced.
Ascospore discharged from leaves are dispersed by wind to expanding floral primordia and unfolding leaves. Floral, leaf, and fruit tissues are much more susceptible when young than when mature. Early infection, particularly of floral structures, by primary inoculum (ascospores) is thus extremely significant in the epidemiology of this disease because the fungus becomes established in a favourable location for secondary infection of the developing fruit and leaves. The critical time for the development of apple scab is from the opening of fruit buds until petal fall. If the disease can be suppressed during this time, its later management is usually easier. Thus, this period is the second key stage in the life cycle for the disease control.
After the host penetrates, a fungal stoma eventually develops between the cuticle and the outer walls of the epidermal cells. This stroma produces conidiophores which rupture the host cuticle. Conidia borne from these conidiophores are dispersed by the movement of wind and rain to susceptible leaves and fruit where secondary infection occurs. This secondary infection repeats itself until leaf fall in the autumn. When the pathogen, V. inaequalis, infects developing fruit, it causes corky lesions and deformations, reducing yields and making fruit unmarketable. The overwintering saprophytic stage is then re-initiated.
Early attempts to use urea to reduce the primary inoculum of V. inaequalis was reported in the 1960's (Cook, 1969). Since then a lot of attention was focused on safe methods for the reduction of the primary inoculum and less attention was devoted to methods which employed dangerous chemicals such as DNOC and lead arsenate. Cultural methods aimed at destroying the leaves such as mulching and tilling were also successfully used in the past.
Certain organisms, mainly fungal isolates, when applied in autumn on fallen leaves will inhibit pseudothecial formation and thus reduce ascospore production in the following spring. Heye (1982) found a fungus, Athelia bombacina, which completely inhibited the formation of pseudothecia on sterile discs (Heye and Andrews, 1983). This antagonist was also tested under field conditions. However, results are incomplete since it was not evaluated during the whole ascospore ejection season (Miedtke and Kennel, 1990).
Thus, there is a need for a biological control agent against V. inaequalis, which can be used on a commercial scale.